The present invention relates to a titanium sheet (or plate) excellent in balance between stamping formability and strength. Specifically, the present invention relates to a pure titanium sheet which includes pure titanium corresponding to Grade 2 prescribed in Japanese Industrial Standards (JIS) H4600 (2007), has a 0.2% yield strength (hereinafter also referred to as “strength”) of 215 MPa or more, and excels in stamping formability.
Titanium, as excelling in properties such as corrosion resistance, specific strength, and lightness in weight, has been adopted to a wide variety of fields and applications such as eyeglass frames; camera casings; cabinets typically of mobile devices; structures such as bicycles; exhaust system parts, such as mufflers, of motorcycles and automobiles; pipes and plates of heat exchangers; and electrodes of chemical plant tanks. Titanium is also expected to be adopted to advanced materials such as separators for fuel cells.
Pure titanium, which is heavily used in these applications, is prescribed in JIS H4600 standard and is categorized typically into JIS Grade 1, Grade 2, and Grade 3 by the contents of impurities such as iron (Fe) and oxygen (O), and the strength. With an increasing grade number, the pure titanium has an increasing minimum strength, and different grades of pure titanium are used in accordance with different purposes.
One of applications in which pure titanium sheets are mostly used is a plate heat exchanger (PHE). Titanium sheets to be adopted to this application are generally subjected to cold stamping into a complicated corrugated shape so as to have a larger heat exchange effective area in order to improve heat exchanger effectiveness. Specifically, they are exposed to a stamping environment which is extremely severe for the material. A pure titanium of JIS Grade 1 which is soft and is most liable to be formed among the grades is used as a pure titanium to be used under such severe stamping conditions.
However, such a titanium sheet requires further higher strength and more excellent formability, because improvements in heat exchanger effectiveness of a heat exchanger are performed not only based on the shape of the heat exchanger itself but also typically based on increase in flow rate of a heat medium (or cooling medium), and such a measure requires higher pressure tightness. Strength and stamping formability are, however, trade-off properties, and the fact is that no titanium sheet which satisfies the requirements in two properties has been proposed.
As techniques for improving stamping formability of pure titanium sheets, there have been proposed a technique of controlling the structure of titanium (Japanese Unexamined Patent Application Publication (JP-A) No. 2004-285457) and a technique of alloying titanium (Japanese Unexamined Patent Application Publication (JP-A) No. 2002-317234). These techniques, however, are intended to improve the formability of a pure titanium having a strength (yield strength) corresponding to JIS Grade 1, but are not intended to improve the formability of a pure titanium having a strength corresponding to JIS Grade 2. Specifically, twin deformation takes a major role in deformation in the pure titanium of JIS Grade 1; but a pure titanium having a strength corresponding to JIS Grade 2 is unlikely to undergo twin deformation and is not improved in formability when the techniques aiming at the pure titanium of JIS Grade 1 are adopted thereto without modification.
As a technique for improving the stamping formability of pure titanium having a strength corresponding to JIS Grade 2 or JIS Grade 3 (215 MPa or more), Japanese Unexamined Patent Application Publication (JP-A) No. 2009-228092 proposes a technique of controlling the contents of oxygen (O) and iron (Fe) and controlling titanium grain size. However, it is difficult to attain good balance between stamping formability and strength merely by controlling oxygen and iron contents and titanium grain size.